The present invention generally relates to semiconductor lasers and more particularly to a semiconductor laser that has a window region of a small quantity of laser light absorption at its light-emitting end surfaces. A semiconductor laser of this type is applied to an optical disk drive and the like that requires a high output.
The present invention also relates to a semiconductor laser fabricating method capable of fabricating the semiconductor laser of the above-mentioned type with high accuracy.
In the high-output semiconductor laser for use in an optical disk drive or the like, the light-emitting end surface sometimes deteriorates due to high density of light, possibly causing damage called COD (Catastrophic Optical Damage). As a measure against this, it has been proposed to provide the light-emitting end surfaces with a window region that absorbs less laser light than the inside of the active layer does.
As a conventional high-output semiconductor laser that has a window region at its light-emitting end surfaces, there is one as shown in FIG. 20 (see WO96/11503). This semiconductor laser has on an n-type GaAs substrate 1 an n-conductivity type buffer layer 11, an n-conductivity type first cladding layer 2xe2x80x2, a first separate confinement layer 2xe2x80x3, an active layer 3, a second separate confinement layer 4xe2x80x3, a p-conductivity type second cladding layer 4xe2x80x2 and an etching stopper layer (having a thickness of 0.01 xcexcm) 5. A p-conductivity type second cladding layer 40, a p-conductivity type intermediate layer 9 and a p-conductivity type first contact layer 10 are provided on this etching stopper layer 5 so as to constitute a mesa 12 that extends in a striped shape in the direction of line XXIxe2x80x94XXI in FIG. 20. Regions at both sides of the mesa 12 are filled with an n-type current blocking layer 13. A second contact layer 6 and an electrode (connection conductor) 7 are provided over the mesa 12 and the n-type current blocking layer 13. On the other hand, an electrode (connection conductor) 8 is formed over the rear surface of the n-type GaAs substrate 1.
As shown in FIG. 21 (showing a cross section taken along the line XXIxe2x80x94XXI of FIG. 20), the active layer 3 is constructed of a laminate of two quantum well layers 3xe2x80x2 and a barrier layer 3xe2x80x3 therebetween. Portions, which belong to the active layer 3 and are located near light-emitting end surfaces (exit surfaces) 50 and 51, serve as window regions (passive regions) 3B where the laser light absorption is less than in the active layer inside 3A.
This semiconductor laser is fabricated as follows. As shown in FIG. 22, the layers of the n-conductivity type buffer layer 11 through the contact layer 10 are first grown on the n-type GaAs substrate 1 by OMVPE (organometallic vapor phase epitaxy). Next, a masking layer 30 made of silicon oxide is formed so as to have opening portions 31 and 32 along the light-emitting end surfaces 50 and 51. The wafer in this state is introduced in a closed capsule together with zinc arsenide and the capsule is heated to a temperature of 600xc2x0 C., so that Zn atoms 59 diffuse from the upper surface side of the contact layer 10 beyond the active layer 3. Through these processes, local intermixing of the active layer 3 (namely making a part of the active layer 3 a mixed crystal) takes place at the portions near the light-emitting end surfaces 50 and 51, which serve as the window regions 3B where the energy bandgap is greater and accordingly the laser light absorption is less than in the active layer inside 3A. After the mask 30 is removed, a strip-shaped mask 40 is formed, which extends perpendicularly to the light-emitting end surfaces 50 and 51, as shown in FIG. 23. Next, the mesa 12 is formed just under the mask 40 by etching the semiconductor layers 10, 9, and 40 at portions on both sides of the mask 40 until the etching stopper layer 5 is reached. Subsequently, as shown in FIG. 20, the blocking layer 13 is formed on both sides of the mesa 12 by OMVPE. After planarizing the blocking layer and removing the mask 40, the second contact layer 6 is formed by using the OMVPE method again. Then, the electrodes 7 and 8 are formed over the upper surface of the contact layer 6 and the lower surface of the substrate 1, respectively (the fabrication completed).
According to the aforementioned fabricating method, during the step of forming the window regions (passive regions) 3B through intermixing of the active layer 3 by diffusion of impurity, intermixing of the etching stopper layer 5 may also take place. Then, there will be a problem that the etching stopper layer 5 and the second cladding layer (lower portion) 4xe2x80x2 are etched in the process of forming the mesa 12, which leads to reduction of the processing accuracy of the mesa 12. If the etching progresses extremely, there may arise a further problem that the current blocking layer 13 and the n-type cladding layer 11 are disadvantageously electrically short-circuited. On the other hand, if the annealing temperature and time are reduced to avoid these problems related to the fabricating process, then there may conversely arise a problem that sufficient intermixing does not take place in the window region 3B, resulting in difficulties in obtaining the effect of restraining photoabsorption.
Accordingly, it is an object of this invention to provide a semiconductor laser which has a window region in its light-emitting end surfaces and is able to be easily fabricated with high accuracy.
Another object of this invention is to provide a method for easily fabricating a semiconductor laser having a window region in its light-emitting end surfaces, with high accuracy.
In order to accomplish the first object, there is provided, according to an aspect of the present invention, a semiconductor laser, which emits laser light through a light-emitting end surface, comprising:
a lower cladding layer, an active layer for generating laser light, a first upper cladding layer and an etching stopper layer stacked in this order on a substrate;
a second upper cladding layer formed in a shape of a ridge on the etching stopper layer, the ridge extending perpendicularly to the light-emitting end surface;
a current blocking layer disposed in regions on both sides of the second upper cladding layer; and
an impurity diffused in a portion extending along the light-emitting end surface from the etching stopper layer to the active layer and located at least under the ridge for local intermixing in this portion to restrain laser light absorption, wherein
in a region along the light-emitting end surface, the etching stopper layer has a bandgap smaller in portions thereof disposed in positions corresponding to both sides of the ridge than in a portion thereof located just under the ridge.
In the semiconductor laser of the present invention, in the region along the light-emitting end surface, the energy bandgap of the portions, of the etching stopper layer, that correspond to both sides of the ridge is smaller than the energy bandgap of the portion, of the etching stopper layer, that is located just under the ridge. Therefore, in the region along the light-emitting end surface, the portions corresponding to both sides of the ridge of the etching stopper layer can effectively fulfill the function to stop the etching when the second upper cladding layer is formed in a ridge shape on the etching stopper layer. Therefore, this semiconductor laser is easily fabricated with high accuracy.
In one embodiment, in the region along the light-emitting end surface, the active layer has a bandgap larger in a portion thereof located just under the ridge than in portions thereof disposed in positions corresponding to both sides of the ridge.
Accordingly, the portion, which belongs to the active layer and is located just under the ridge in the region along the light-emitting end surface, can effectively restrain the COD, serving as a window region. Moreover, because intermixing does not take place in an internal region of the active layer, the fabrication becomes easy. It is to be noted that the problem of COD does not occur in the internal area of the active layer, so that the internal area of the active layer is not required to be intermixed.
In one embodiment, in the region along the light-emitting end surface, a photoluminescence wavelength shift to a shorter wavelength side due to the local intermixing of the active layer in the portion located just under the ridge is 18 nm or more. Therefore, the maximum optical output is increased by 1.41 times or more in comparison with the conventional semiconductor laser. Moreover, a photoluminescence wavelength shift to the shorter wavelength side due to the local intermixing of the active layer in the portions corresponding to both sides of the ridge is not larger than 15 nm. Therefore, fabrication becomes extremely easy.
In one embodiment, the first upper cladding layer contains a diffused impurity of Be or C, and the impurity diffused in said portion extending along the light-emitting end surface from the etching stopper layer to the active layer is Zn.
The Zn atoms easily diffuse, and the diffusion easily causes the intermixing of the active layer. Moreover, the elements Be and C have diffusion constants smaller than that of Zn. Therefore, the Zn atoms can easily be diffused into the active layer while avoiding the phenomenon of the diffusion of the diffused impurity (Be or C) contained in the first upper cladding layer into the active layer. Therefore, the semiconductor laser is easy to fabricate.
In one embodiment, the second upper cladding layer contains a diffused impurity of Be or C.
The elements of Be and C have diffusion constants smaller than that of Zn. Therefore, Zn atoms can easily be diffused into the active layer while avoiding the phenomenon of the diffusion of the diffused impurity (Be or C) contained in the second upper cladding layer into the active layer. Therefore, the semiconductor laser is easy to fabricate.
In one embodiment, the active layer comprises at least one quantum well layer and barrier layers alternating with the quantum well layer. The at least one quantum well layer is constructed of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61), and the barrier layers are constructed of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61) whose Al content (x) is greater than that of the quantum well layer.
The substance of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP that constitutes the quantum well layer and the barrier layers is easily intermixed even when the concentration of Zn atoms to be diffused is on the order of a comparatively low value of 1018 cmxe2x88x923. Therefore, the semiconductor laser is easy to fabricate.
It should be understood that although the letters of x, y and z are herein used for expressing the compositions of the compound semiconductors, x, y and z can take different values in each of the compound semiconductors.
In one embodiment, the etching stopper layer is constructed of GayIn1xe2x88x92yP (0xe2x89xa6yxe2x89xa61), and the first and second upper cladding layers are each constructed of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61).
The substance of GayIn1xe2x88x92yP (0xe2x89xa6yxe2x89xa61), which constitutes the etching stopper layer, allows the substance of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61), which constitutes the first and second upper cladding layers, to be selectively left when the latter substance is removed by wet etching. Therefore, the second upper cladding layer is easily formed in a ridge shape on the etching stopper layer.
In one embodiment, the active layer comprises at least one quantum well layer and barrier layers alternating with the quantum well layer. The at least one quantum well layer is constructed of InzGa1xe2x88x92zAs (0xe2x89xa6zxe2x89xa61) or AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61), and the barrier layers are constructed of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61) whose Al content (x) is greater than that of the quantum well layer when the latter is constructed of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61).
The substance of InzGa1xe2x88x92zAs (0xe2x89xa6zxe2x89xa61) and AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61), either of which constitutes the quantum well layer or layers, and the substance of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61), which constitutes the barrier layers, are both easily be intermixed by the diffusion of Zn atoms. Therefore, the semiconductor laser is easy to fabricate.
In one embodiment, the etching stopper layer is constructed of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa60.3), and the first and second upper cladding layers are each constructed of AlyGa1xe2x88x92yAs (x less than yxe2x89xa61).
The substance of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa60.3), which constitutes the etching stopper layer, allows the substance of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61), which constitutes the first and second upper cladding layers, to be selectively left when the latter substance is removed by wet etching. Therefore, the second upper cladding layer is easily formed in a ridge shape on the etching stopper layer.
In order to accomplish the second object, there is provided, according to another aspect of the present invention, a method for fabricating a semiconductor laser that emits laser light through a light-emitting end surface, comprising:
a process for forming at least a lower cladding layer, an active layer for generating laser light, a first upper cladding layer, an etching stopper layer and a second upper cladding layer in this order on a substrate;
a first annealing process for diffusing an impurity for restraining laser light absorption into the second upper cladding layer along a region where a light-emitting end surface is to be formed, under a condition that allows the etching stopper layer to maintain a function of stopping etching for the second upper cladding layer;
an etching process for performing etching until the etching stopper layer is reached such that the second upper cladding layer is left in a ridge shape extending perpendicularly to the light-emitting end surface to be formed; and
a second annealing process for re-diffusing the impurity once diffused in the region of the ridge-shaped second upper cladding layer where the light-emitting end surface is to be formed, into the active layer through the etching stopper layer to thereby cause local intermixing of the active layer in a portion that extends along the light-emitting end surface to be formed and is located just under the ridge.
According to the semiconductor laser fabricating method of the present invention, as shown by example in FIG. 18A, at least a lower cladding layer 71, an active layer 72 for generating laser light, a first upper cladding layer 73, an etching stopper layer 74 and a second upper cladding layer 76 are stacked in this order on a substrate. The first annealing process is carried out under the condition that allows the etching stopper layer to maintain the function of stopping the etching for the second upper cladding layer, or for example, under the condition of a low temperature or a short time. As shown by example in FIG. 18B, an impurity 89 is diffused from, for example, a solid diffusion source 81 to the second upper cladding layer 76, whereas the impurity is substantially not diffused to the etching stopper layer 74. Therefore, in the etching process for processing the second upper cladding layer 76 in a ridge shape, the etching stopper layer 74 can effectively stop the etching as shown by example in FIG. 18C. As a result, the processing accuracy of the ridge is increased, and the electrical short-circuit between layers is prevented. Moreover, in the second annealing process, as shown by example in FIGS. 19A and 19B, sufficient local intermixing of the active layer takes place in a portion 72B that extends along the light-emitting end surface to be formed and is located just under the ridge 76. This portion 72B where the intermixing took place operates as a window region absorbing little laser light at the light-emitting end surface after the completion of the semiconductor laser, allowing the COD to be restrained. The semiconductor laser fabricating method with the above-mentioned arrangement can easily fabricate a semiconductor laser having a window region at the light-emitting end surface with high accuracy.
In one embodiment, a photoluminescence wavelength shift to a shorter wavelength side through the first annealing process at a portion of the active layer that extends along the region where the light-emitting end surface is to be formed is not larger than 15 nm. Therefore, intermixing of the etching stopper layer hardly occurs in the first annealing process. Therefore, after the first annealing process, the function of the etching stopper layer to stop the etching for the second upper cladding layer can be maintained. Furthermore, a photoluminescence wavelength shift to the shorter wavelength side through the second annealing process at the portion of the active layer that extends along the region where the light-emitting end surface is to be formed and is located just under the ridge is 18 nm or larger. This means that sufficient local intermixing of the active layer took place at that portion in the second annealing process. This portion in which intermixing took place operates as a window region of a small quantity of laser light absorption at the light-emitting end surface after the completion of the semiconductor laser, allowing the COD to be restrained.
In one embodiment, after the etching process and before the second annealing process, the method further includes providing on the substrate an impurity evaporation preventing layer for preventing the impurity from evaporating from the second upper cladding layer to the outside.
In the state in which nothing is provided on the second upper cladding layer as shown by example in FIG. 19A, the impurity 89 will be partially evaporated from the second upper cladding layer 76 to the outside during the second annealing process. In contrast to this, as shown by example in FIG. 19B, according to the semiconductor laser fabricating method of this embodiment, the second annealing process is carried out in a state in which an impurity evaporation preventing layer 85 for preventing the evaporation of the impurity 89 from the second upper cladding layer 76 to the outside is provided. As a result, the evaporation of the impurity 89 from the second upper cladding layer 76 to the outside can be prevented. Therefore, the intermixing through the second annealing process of the active layer in the portion 72B, which extends along the region where the light-emitting end surface is to be formed and is located just under the ridge, can be further promoted.
In one embodiment, the impurity evaporation preventing layer is made of silicon oxide, silicon nitride, or alumina.
The substance of silicon oxide, silicon nitride or alumina is fine and dense and therefore suitable for preventing the evaporation of the impurity. Moreover, the substance of silicon oxide, silicon nitride or alumina can selectively be removed by an etchant that does not erode a semiconductor underlayer. Therefore, this method can easily fabricate the semiconductor laser with high accuracy.
Generally in the semiconductor laser, a compound semiconductor layer, such as a current blocking layer or the like, is formed on the etching stopper layer. In one embodiment, such a compound semiconductor layer is utilized as the impurity evaporation preventing layer. Therefore, the fabricating process can be simplified.
In one embodiment, the conductor semiconductor layer serving as the impurity evaporation preventing layer is of a conductivity type different from that of second upper cladding layer. The conductor semiconductor layer can be utilized to form a current blocking layer for restraining a wattless current.
In one embodiment, the impurity evaporation preventing layer is formed at a temperature lower than a temperature at which the impurity is re-diffused in the second annealing process.
According to the semiconductor laser fabricating method of this embodiment, the impurity is prevented from evaporating during formation of the impurity evaporation preventing layer, due to the temperature at which the impurity evaporation preventing layer is formed.
In one embodiment, the first upper cladding layer contains a diffused impurity of Be or C, and the impurity diffused in the portion of the active layer that extends along the region where the light-emitting end surface is to be formed and is located just under the ridge is Zn.
In one embodiment, the diffused impurity contained in the second upper cladding layer is Be or C.
In one embodiment, the active layer is formed by alternating at least one quantum well layer and barrier layers. The at least one quantum well layer is constructed of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61), the barrier layers are constructed of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61) whose Al content (x) is greater than that of the quantum well layer. The first annealing process is carried out under a condition of a temperature of 450xc2x0 C. to 570xc2x0 C. for 10 minutes or more or a temperature of 550xc2x0 C. to 650xc2x0 C. for 10 minutes or less. And, the second annealing process is carried out under a condition of a temperature of 570xc2x0 C. to 750xc2x0 C. for 10 minutes or more or a temperature of 650xc2x0 C. to 850xc2x0 C. for 10 minutes or less.
Under the condition of the first annealing process, when the etching stopper layer is constructed of GayIn1xe2x88x92yP (0xe2x89xa6yxe2x89xa61), intermixing of the etching stopper layer does not take place. Moreover, under the condition of the second annealing process, the active layer is satisfactorily activated. However, under a temperature condition that is higher than the temperature condition of the second annealing process, the diffused impurity (p-type, in particular) in the second upper cladding layer disadvantageously diffuses into regions of the active layer other than the region near the light-emitting end surface, possibly deteriorating the characteristics of a laser oscillation threshold value and so on.
The first annealing process under the conditions of a temperature of 450xc2x0 C. to 570xc2x0 C. and ten minutes or more can be carried out by using an ordinary annealing furnace, while the first annealing process under the conditions of a temperature of 550xc2x0 C. to 650xc2x0 C. and ten minutes or less can be carried out by using the RTA (Rapid Thermal Annealing) method.
Molecular beam epitaxy is a semiconductor forming technique to be carried out in a high vacuum without using hydrogen. Therefore, if the second annealing process is carried out by raising a substrate temperature when a semiconductor layer is formed by molecular beam epitaxy, then the diffused impurity inactivation due to the mixture of hydrogen can be restrained.
In one embodiment, the second annealing process is carried out in a nitrogen ambient. According to this embodiment, the diffused impurity inactivation due to the mixture of hydrogen can be restrained.